1. Field of the Disclosure
This disclosure relates to semiconductor memory devices, and, more particularly, to semiconductor random access memory devices that utilize a magnetic field.
2. Description of the Related Art
Non-volatile memory devices are an important component in electronic systems. Flash memory is the major non-volatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Disadvantages of Flash memory include high voltage requirements and slow program and erase times. Also, Flash memory has a poor write endurance before memory failure. In addition, to maintain reasonable data retention, the scaling of the gate oxide is restricted by the tunneling barrier seen by the electrons. Hence, Flash memory is limited in the dimensions to which it can be scaled.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive random access memory (hereinafter referred to as “MRAM”). To be commercially practical, however, MRAM should have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the nonvolatile memory state, the repeatability of the read/write cycles, and the memory element-to-element switching field uniformity are three aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive differences in the MRAM device between the two states. The magnetic fields for writing are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
A typical embodiment of an MRAM circuit involves, but is not limited to, having each bit in proximity to two lines that are orthogonal to one another, with one line being below the bit and one line being above the bit. This grouping of bit and two lines will hereafter be referred to as a “bit cell”. Arraying the bit cells into a grid pattern causes one line to form columns (hereafter referred to as the “bit line” and the other line to form rows (hereafter referred to as the “digit line”). The MRAM device can be written using two distinct modes; a conventional write mode or a toggle write mode.
In the conventional write mode, the state of the MRAM cell is switched only if the state that is written is different than the state that is stored, and the polarity of the current in one or both of the digit or bit lines determines the memory state that is written. In the conventional write mode, it is necessary that at least one of the write lines be capable of passing current in two directions. In one embodiment, field generated by the digit line in a given bit cell reduces the energy barrier to reversal for the bit in that cell. Field generated by a bit line in this cell may then reverse the direction of the magnetic moment of the bit if it is in the opposite direction to the magnetic moment of the bit before field application and greater than the now-reduced energy barrier.
In the toggle write mode, the state of the MRAM cell is switched every time a sequence of strategically-timed pulses is generated from both the bit and digit lines. In the toggle write mode, it is not necessary that either of the write lines be capable of passing current in two directions. Since the memory state changes with each application of the write pulse sequence, the initial state of the MRAM cell is determined prior to each write. After comparing the stored memory state with the new state to be written, the MRAM device is written if the stored state and the new state are different. The toggle write mode is disclosed in U.S. Pat. No. 6,545,906 (herein after '906) entitled “METHOD OF WRITING TO SCALABLE MAGNETORESISTANCE RANDOM ACCESS MEMORY ELEMENT”, issued to Savtchenko et al. on Apr. 8, 2003 and incorporated herein by reference in its entirety.
In one implementation of the conventional switching mode, the digit line provides a “half-select” field to a row of bits within an array, reducing the energy barrier to reversal for the entire row. A bit line is simultaneously energized, selecting the bit at the intersection of the energized bit line and energized digit line for writing. The bit line current, Ibit, necessary to switch the bit at a given digit line current, Idigit, is defined as Isw(Idigit). Isw(Idigit) is lowered as Idigit is increased. Isw(Idigit) for bits within an array follows a gaussian distribution, where σsw(Idigit) is the one-sigma switching distribution of the bits at a given Idigit. Therefore, the MRAM device must supply approximately Isw(Idigit)+6*σsw(Idigit) to write the bits with an error rate of about 1 in 109 bits swithching unreliably. The non-half-selected switching field of a given bit is Isw(0). It is necessary that Isw(Idigit)+6*σsw(Idigit)<Isw(0)−6*σsw(0) or disturb errors will occur along the bit line. It is also necessary that Idigit be low enough such that Isw(Idigit)−6*σsw(Idigit)>0 or disturb errors will occur along the digit line.
A typical implementation of the toggle switching mode is similar to the conventional switching mode in that the bits are in an array with orthogonal bit line and digit lines. However, in this case there is no “half-select” upon the application of a single field. Both lines work together in a strategically-timed sequence to rotate the magnetic moment of the bit 180 degrees with each application of write pulses. The current required to switch MRAM bits follows a gaussian distribution for each write line. The MRAM device must supply approximately 6*σsw over the mean switching current (Isw) on both lines. There is also a point on each line where too much field has been applied and the bit no longer switches reliably. This is the saturation current, or Isat. Therefore, to ensure that less than 1 in 109 bits switches unreliably, the following condition must be met for both lines: Isw+6*σsw<Isat−6*σsat.
Due to the aforementioned constraints, or “windows”, in reliably switching both conventional-mode and toggle-mode MRAM, it is disadvantageous to apply more current during a write than is absolutely necessary to switch the bits. There are several other reasons to reduce the write currents as well. For instance, a half-selected bit sharing a row with and neighboring a selected bit may experience enough field to switch if the half-selected bit switches in the lower end of the switching distribution and the write current is too high. This is known as a “neighboring bit disturb” and can be alleviated by reducing the over-drive write current by reducing σsw or the number of σsw over Isw that is applied. Also, since the power consumed during a write is the resistance of each line times its write current squared, a small reduction in write current equates to a large reduction in power used. In addition, electromigration effects increase with current magnitude, so reducing the currents through the write lines improves part reliability.
The use of the same reference symbols in different drawings indicates similar or identical items.